The present invention relates to an alkaline solution developable liquid image-producing composition useful as a permanent protective layer for printed circuit boards i.e., a solder mask.
Solder mask is used as a covering layer on a printed circuit board for preventing short circuiting during the soldering process, protecting the metallic wires from corrosion, and maintaining good electrical insulation. The performance of a solder mask is determined by its heat resistance, solvent resistance and adhesiveness. These properties are generally determined as follows: A test plate coated with the solder mask is dipped into a molten tin-lead solder at 240.degree.-280.degree. C. for 10 seconds and then into dichloromethane for 2 minutes. An adhesiveness test is then carried out with a No. 600 adhesive tape available from 3M corporation, USA. Although peeling of the antisoldering layer may be observed, qualified solder masks should not show in any trace of peeling under such test.
To meet the standards for finer and closer wire lines on the printed circuit boards, photographic film-forming solder mask has been developed. Such solder mask is a photopolymerizable composition which can be polymerized by irradiation with light of a specific wavelength range. If a mask covers the composition, the non-irradiated areas of the composition will not polymerize and therefore can be subsequently removed by dissolving with a solvent or an alkaline solution. An image of polymerized antisoldering layer is then obtained. The image can then be further irradiated with ultraviolet light or heated to about 150.degree. C. to give a stronger protective layer.
Photographic solder masks can be classified into dry film and liquid types. European Patent Application No. EP 0 115 354 discloses a dry film type solder mask in which a photopolymerizable composition is inserted between a polyester film and a polyvinyl ester film. When the film is utilized, the polyvinyl film is removed and the remaining layer is pressed onto a printed circuit board. The disadvantage of the film is that poor adherence is obtained between the wired lines. Worse results are frequently found when the gap between wire lines is below 8 mil.
Liquid photographic solder mask, on the contrary, possesses superior adhesiveness due to its better flowability and wettability to the substrate.
Photographic solder mask can be developed with solvent. However, the utilization of solvent causes environmental problems and solvent recovery further adds to the cost.
Aqueous alkaline solutions such as 1% aqueous Na.sub.2 CO.sub.3 solution, have also been used for developing photographic solder mask. If the aqueous alkaline solution is used in combination with photographic solder mask having a solid content higher than 70 wt%, environmental pollution problems can be significantly alleviated.
Conventionally, an alkaline solution developable solder mask is composed of an acid group containing straight chain polymeric binder, photo- or heat-sensitive crosslinkable monomers and a photoinitiator. Similar solder masks have been disclosed in, for example, European Patent Application No. EP 0 115 354 and Japanese Laid-Open Patent Application No. Sho 62-160440. The acidic binder does not participate in the photo-sensitive or heat-sensitive crosslinking reaction, and thus although the composition may be developed with weak alkaline solution, the resultant crosslinked film after baking at higher temperature cannot resist the corrosion of strong alkaline solution, such as 5% NaOH. The other properties such as solvent resistance (e.g. against CH.sub.2 Cl.sub.2), thermal resistance and impact strength are also inferior. The shortcomings of the properties are caused by high solubility of the acidic binder and the weak crosslinked structure of the small molecular weight monomer.